It isn't every book whose author thanks in his acknowledgments the agent and editor who didn't think him mad and the wife who did. In fact, this divided verdict makes a sort of sense. Aliens is a peculiar project, gradually earning back in its second half the respect it fritters away in the first.

The respectable approach to the subject of extraterrestrial life is to sift testimonies as rigorously as possible, disregarding anything suspect or uncorroborated. Bryan Appleyard does something very different: he treats phenomena as equally significant in cultural terms, whether they are 'real' or not.

Along the way, he almost insists on being dismissed as credulous, but he never quite lets go of his scepticism. He refers in his introduction to having seen 'an alien spaceship in broad daylight in Norfolk', but he was in a deep hypnotic trance in London at the time (for research purposes) and isn't actually claiming a sighting.

The first sentence reads: 'Since 1947, aliens have poured from the abyss that lies between ourselves and the world.' Coupled with his watch-the-skies subtitle, Why They Are Here, and the witty cover design (of an alien with huge eyes and slit mouth sitting at a desk wearing a shirt and tie), this seems a declaration of belief in the reality of visitors from other worlds.

But look at the sentence again: that abyss isn't deep space but something that lies between us and the world. It is a sense of unease, of not belonging, the necessary state of an animal at odds with its instincts, whose intelligence as it reaches outwards finds evidence only of its insignificance.

The truths of the scientific project, whether unearthed by Copernicus, Darwin or Freud, humble rather than exalt us. It is we who are alien to the world which sustains us and we project that estrangement on to the cosmos.

Appleyard has difficulty settling on a tone, particularly in the early part of the book. When he quotes a 'top-secret Swedish report' from November 1948 to the effect that 'ghost rockets' seen in the sky were not from earth, without wondering how a top-secret source comes to be common property (or offering any sort of source for it), he seems to be playing the game of sensationalism.

His sense of historical context is certainly slapdash - 'The bikinis and washing machines were apples in the new Eden. The Bomb was God's judgment. We ate and found ourselves evicted into a blasted, alien landscape of ultimate risk.' That's the Fifties. Guess what happened in 1979? That's right: 'The summer of love was replaced by the winter of discontent.'

Chapters end with tabloid-style teasers: 'Why were they so elusive? Why weren't they making contact as they did in the movies? Why weren't they talking to us? In fact, they were.' At other times, the style is almost comically flat ('Nuclear fallout represents one form of anxiety; an anal probe another').

The effort to appeal to two audiences, the committed and the sceptical, may end up with the alienation of both. True believers are likely to find the book not only heavy going, when quotations from Wallace Stevens and John Ashbery are recruited to raise the tone, but something of a betrayal.

Appleyard says he moved during the writing of the book 'from scepticism to belief and finally came to rest with acceptance'. By this, he means that he chooses to accept aliens 'for what they are - essential expressions and necessary aspects of the eternal and irresolvable predicament of self-consciousness'.

Aliens is better constructed than it is written. Gradually, the focus moves from eye witnesses to imaginative writers, to scientists and to philosophers. Although it's silly to describe Galileo as 'a cyborg' because he applied a telescope to his eye and looked at the heavens, it's astute to pinpoint the launch of the Apple Mac with its user-friendly layout as a moment of change. The innovation of desktop and mouse 'did not require you, as all previous machines had done, to talk machine language; it required the machine to pretend to talk yours'. We don't know what is going on behind the screen and as Appleyard says: 'Concealment - a more negative but also more apposite word than interiority - is a first step towards personhood.'

The chapter on Philip K Dick (chastised for the sloppiness of his writing!) does not do justice to someone who was both a delusional psychotic and an extraordinary distiller of visions. Appleyard makes a good case, though, for Stanislaw Lem, not only as a writer but a thinker about what non-human experience would be like. Lem's message is that if we ever encounter aliens, 'they will not deliver rebukes for our aggression, warnings about the environment or cosy spiritual sermons. They will not abduct and interbreed with us. They will not control our minds with the steady gaze of their enormous black eyes. They will not even attack us with their rayguns or disruptor beams'.

Whatever their level of technological development, they won't have the power to rescue us from our loneliness, to resolve the tangle of emotions which results from our longing for company and fear of the Other.

Does it matter if aliens are 'real' or not if their existence, and even their presence, would make no difference to our place in the cosmos? At moments like these, Appleyard's curious, cranky book seems to be on to something.